1. Field of the Invention
The present invention relates to semiconductor devices and components. Particularly, the present invention relates to forming passive elements, such as inductors, resistors, and capacitors, wherein the passive elements have relatively precise electrical properties.
2. Background of Related Art
Conventional screen-printed resistors, which may be termed “thick-film” resistors, are employed in hybrid electronic circuits to provide a wide range of electrical resistance values. Conventional screen-printing processes are used to deposit conductive paste or ink upon a surface of a substrate, such as a substrate comprising FR-4, flexible circuit, ceramic, or silicon. Screen-printing pastes used with ceramic printed wire boards may typically include a glass frit composition, an electrically conductive material, various additives for favorably affecting the electrical properties of the resistor, and an organic vehicle or polymer matrix material. Screen-printing pastes used in organic printed wire board construction typically include an electrically conductive material, various additives for favorably affecting the electrical properties of the resistor, an organic binder, and an organic vehicle. After printing, the screen-printing paste may be typically heated to dry the paste and convert the paste into a suitable film that adheres to the substrate. If a polymer screen-printing paste is used, the heating step may remove the organic vehicle and cure the polymer matrix material. Other screen-printing pastes may be preferably sintered, or fired, during which the paste is heated to burn off the organic vehicle and fuse the remaining solid material.
The electrical resistance of a screen-printed resistor may be dependent, at least partially, on the precision with which the dimensions of the resistor are produced, the stability of the resistor material, and the stability of the resistor terminations. Accuracy in forming of at least one dimension (e.g., a width, length or thickness) of a screen-printed resistor may be particularly challenging in view of the conventional techniques employed, as well as the dimensional instability that may occur during subsequent processing.
Initially, for rectangular screen-printed resistors, the width and thickness are determined by the screen-printing process, and the length is determined by the termination pattern. More particularly, conventional screen-printing techniques generally employ a template with apertures bearing the positive image of the resistor to be created. The template, referred to as a mask or stencil, may be placed proximate to and above the surface of the substrate on which the resistor is to be formed. The stencil may then be loaded with the conductive paste, and a so-called squeegee blade may be drawn across the surface of the mask, pressing the paste through the apertures of the stencil and onto the surface of the substrate.
However, even if the dimensions of a conventional screen-printed passive element are reasonably well controlled upon initially depositing the paste upon the surface of a substrate, the control of dimensions may be influenced by dimensional changes that occur after deposition (i.e., during drying, firing, or both drying and firing). Of course, such dimensional changes may be difficult to predict or control and may adversely influence the variability in the electrical properties of a screen-printed passive element. Thus, as mentioned above, compared to many other deposition processes, conventional screen printing is a relatively imprecise process with respect to dimensional tolerances. Accordingly, since the resistance of a screen-printed resistor is related directly to its dimensions, the resistance of a screen-printed resistor or another electrical component may be, correspondingly, relatively imprecise.
For instance, screen-printed resistors may exhibit dimensional tolerances of about ±100 μm. Correspondingly, screen-printed resistors may be typically limited to dimensions of larger than about one square millimeter, since the resistance of a screen-printed resistor of about one square millimeter may generally vary by about 20% to 30% if formed by screen printing, due, in large part, to the variability of its length, width, and thickness. Accordingly, screen-printed resistors which exhibit adequate tolerances in resistance may require the physical size of the resistor to be larger than would otherwise be desirable. Thus, variability with respect to the electrical properties (i.e., resistance) of less than ±20% may be difficult to achieve by conventional screen-printing methods for passive elements having an area of less than about one square millimeter.
For this reason, laser trimming is widely used to adjust the resistance of screen-printed resistors. Laser trimming processes typically involve ablation of a portion of the screen-printed resistor, which increases the electrical resistance thereof. However, laser trimming may be cost prohibitive and may require additional processing time. Also, the resistor must generally be exposed at a surface thereof to allow for laser trimming. As another consideration, resistors which have resistances which exceed a desired magnitude may not be adjusted via laser trimming techniques.
Thus, considering the conventional processes and limitations thereof, undesirably, resistors or other passive electrical components formed on the surface of a substrate via conventional screen-printing processes may occupy a relatively large area on the surface thereof. Limiting the available area may detrimentally influence placement of other circuit components, which may require surface mounting. Therefore, conventional formation of passive elements on the surface of a substrate may be an impediment to design flexibility.
A number of approaches for increasing the accuracy of screen-printed resistors have been developed. For instance, U.S. Pat. Nos. 6,229,098 and 6,171,921 to Dunn et al. each disclose a process for forming a screen-printed resistor with relatively precisely controlled dimensions, thus yielding a relatively precise resistance value. More particularly, U.S. Pat. Nos. 6,229,098 and 6,171,921 to Dunn et al. each disclose that an opening may be photodefined in the surface of a photoimageable layer and then filled via screen printing with resistive material. However, photoimaging processes may be costly, time consuming, or both.
From the above, it can be seen that conventional processes and practices with respect to the fabrication of screen-printed resistors and other passive electrical elements may necessitate a compromise between the precision of the resistance value and the size of the resistor. In other words, while smaller resistors are often preferred to yield a more compact circuit, an undesirable consequence is that resistance values are less predictable due to the dimensional variability thereof. Accordingly, a need exists for a method for producing passive elements that overcomes some of the difficulties associated with conventionally formed passive elements.